[Graph]: fix a minor bug in scan statistics.
[Graph]: fix a minor bug in scan statistics.

It's wrong to count the number of joined vertices in the previous
version. However, the bug didn't cause any problem because each time
only a vertex is passed to the current vertex.

icoming/FlashGraph,zheng-da/FlashX,flashxio/FlashX,zheng-da/FlashX,zheng-da/FlashX,icoming/FlashX,icoming/FlashX,icoming/FlashGraph,icoming/FlashX,icoming/FlashX,icoming/FlashGraph,flashxio/FlashX,zheng-da/FlashX,silky/FlashGraph,flashxio/FlashX,flashxio/FlashX,flashxio/FlashX,silky/FlashGraph,silky/FlashGraph,flashxio/FlashX,icoming/FlashGraph,zheng-da/FlashX,silky/FlashGraph,icoming/FlashGraph,icoming/FlashX,silky/FlashGraph